A stent is commonly used alone or in conjunction with angioplasty to ensure patency through a patient's stenosed vessel. Stents overcome the natural tendency of the vessel walls of some patients to restenose after angioplasty. A stent is typically inserted into a vessel, positioned across a lesion, and then expanded to create or maintain a passageway through the vessel, thereby restoring near-normal blood flow through the vessel.
A variety of stents are known in the art, including self-expandable and expandable stents, as well as wire braid stents. One such stent is described, for example, in U.S. Pat. No. 4,733,665 to Palmaz. Expandable stents are typically delivered to treatment sites on delivery devices, such as balloon catheters or other expandable devices. Balloon catheters may comprise a balloon having a collapsed delivery configuration with wings that are wrapped and folded about the catheter. An expandable stent is then disposed over the balloon and crimped into a collapsed delivery configuration about the balloon by compressing the stent onto the balloon. The stent and balloon assembly may then be delivered, using well-known percutaneous techniques, to a treatment site within the patient's vasculature, for example, within the patient's coronary arteries. Once the stent is positioned across a lesion at the treatment site, it is expanded to a deployed configuration by inflating the balloon. The stent contacts the vessel wall and maintains a path for blood flow through the vessel.
Significant difficulties have been encountered during stent delivery and deployment, including difficulty in maintaining the stent on the balloon and in achieving symmetrical expansion of the stent when deployed. In positioning a balloon expandable stent on the delivery catheter over the fluid expandable balloon, the stent must be smoothly and evenly crimped to closely conform to the overall profile of the catheter and the unexpanded balloon. It has been noted that, due to physical properties of the material used in manufacturing the stent (e.g. stainless steel, tantalum, niobium, platinum, cobalt, chromium or alloys thereof) there is a certain amount of “recoil” of the stent despite the most careful and firm crimping. That is the stent evidences a tendency to slightly open up from the fully crimped position and once the crimping force has been released. For example, in the typical stent delivery and deployment assembly, if the stent has been fully crimped to a first diameter, the stent has been observed to open up or recoil to a second diameter which is approximately 1% to 10% greater than the first diameter. This phenomenon has been characterized as “crimping recoil”. Due to crimping recoil to this slightly enlarged diameter, it can be understood that the stent tends to evidence a certain amount of looseness from its desired close adherence to the overall profile of the underlying catheter and balloon. That is, the stent tends to have a perceptible relatively slack fit in its mounted and crimped position. During delivery, the stent can thus tend to slip and dislocate from its desired position on the catheter or even become separate from the catheter, requiring further intervention by the physician.
The degree of crimping recoil highly depends on the stent material as well as the stent design itself. Presently there is a desire to manufacture stents not only from stainless steel but also from new materials like e.g. cobalt chromium alloys and niobium tantalum alloys. These new materials show increased strength and radiopacity over prior known stent materials and thus allow reduction of wall thickness of the stent struts while the stent's radial force and radio-visibility stays the same. Since reduction of strut thickness has clinically shown to reduce in-stent-restenosis there is a great interest in using these novel stent materials in stent manufacturing. The tradeoff of the increased strength of these alloys is that they often show increased recoil behaviour over the prior known stent materials.
Several techniques have been developed to more securely anchor the stent to the balloon and to ensure more symmetrical expansion. These include plastically deforming the stent so that it is crimped onto the balloon, and sizing the stent such that its internal diameter provides an interference fit with the outside diameter of the balloon catheter. Such techniques have several drawbacks, including less than optimal securement of the stent to the balloon. Consequently, the stent may become prematurely dislodged from the balloon during advancement of the stent delivery system to the treatment site.
Stent delivery systems utilizing a removable sheath disposed over the exterior surface of the stent, which is removed once the stent is positioned at the treatment site, have also been proposed, for example, in U.S. Pat. No. 5,690,644 to Yurek et al. Such systems may be used with or without retainer rings and are intended to protect the stent during delivery and to provide a smooth surface for easier passage through the patient's vasculature. However, the exterior sheath increases the crossing profile of the delivery system while decreasing flexibility, thereby decreasing the ability of the device to track through narrowed and tortuous anatomy.
U.S. Pat. No. 6,106,530 to Harada describes a stent delivery device comprising a balloon catheter having stoppers disposed proximal and distal of a balloon onto which a stent is affixed for delivery. The stoppers are separate from the balloon and maintain the stent's position in relation to the balloon during delivery. As with the removable sheaths discussed previously, the stoppers are expected to increase delivery profile and decrease flexibility of the stent/balloon system.
U.S. Pat. No. 6,110,180 to Foreman et al. provides a catheter with a balloon having pre-formed, outwardly-extending protrusions on the exterior of the balloon. A stent may be crimped onto the balloon such that the protrusions extend into the gaps of the stent, thereby securing the stent about the balloon for delivery. A drawback to this device is the added complexity involved in manufacturing a balloon with pre-formed protrusions. Additionally, if the protrusions are not formed integrally with the balloon, there is a risk that one or more of the protrusions may detach during deployment of the stent. The protrusions may also reduce flexibility in the delivery configuration, thereby reducing ability to track through tortuous anatomy.
U.S. Pat. No. 6,159,227 to Di Caprio et al. provides a catheter with a securement means such as a corrugated tube mounted on the inner shaft underneath the balloon to compensate for crimp recoil and provide increased stent friction. However, similar to what was discussed above with regard to the exterior sheath; the corrugated tube increases the crossing profile of the delivery system while decreasing flexibility, thereby decreasing the ability of the device to track through narrowed and tortuous anatomy.
In each of the devices and methods described above, the balloon is pre-mounted onto a catheter shaft and the stent is then crimped down onto the balloon and catheter shaft. Typically, catheters shafts are constructed of polymeric materials, and thus compress slightly and also re-coil much like the stent does as described above, thereby contributing to the problem.
In view of the drawbacks associated with previously known methods and apparatus for loading a stent onto a stent delivery system, it would be desirable to provide methods and apparatus that overcome those drawbacks.
It would be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that enhance positional stability of the stent during delivery. It would further be desirable to provide methods and apparatus for loading a stent onto a stent delivery system wherein the delivery system comprises a crossing profile and flexibility suitable for use in tortuous and narrowed anatomy. It would still further be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that provide a substantially symmetrical expansion of the stent at deployment.